Dental composites, which essentially comprise a mixture of a polymerisable resin and a glassy filler, have been developing since the early 1970's, when the first materials of this class were introduced. See for example R. L. Bowen et al., “A new series of X-ray-opaque reinforcing fillers for composite materials, J. Dental Research, vol. 51(1) 1972. Until this time, fillings had been based on silver-mercury amalgams, mixtures of acid leachable glass with phosphoric acid (known as “silicate cements”), or unfilled polymerisable resins, and each class of material has certain strengths and weaknesses. For instance, amalgams are generally considered to be cheap and easy to use, and to have a long life time due to their strength and high resistance to wear. Disadvantages of amalgam are toxicity of the mercury and the black colour of the filling. Silicate fillings were approximately tooth coloured and released fluoride into the tooth to help prevent a recurrence of decay. However they tended to dissolve quickly and were weak, and are barely used nowadays. Unfilled resins brought advantages of toughness, convenience, and aesthetics, but were still weak, limiting their use to areas of low stress. These unfilled resins also have a high volume shrinkage, commonly at least 5%. This leads to formation of gaps between the filling and the tooth, and subsequent recurring decay of the tooth around and underneath the restoration. The introduction of composite materials brought improvements in surface hardness, higher physical strengths, good aesthetics, lower shrinkage, and also higher resistance to wear. However the wear rate of these composite materials is still higher than that of amalgam, and their shrinkage of around 2 to 3 volume percent still leads to gap formation and recurrent caries. It is an aim of many researchers in the dental area to develop composite materials with higher strength, reduced shrinkage and higher resistance to wear, which may be used in place of amalgam. Preferably the material should also be extrudable from a dental syringe since this procedure is not only convenient and time saving for the dentist, but also helps to avoid the inclusion of air bubbles in the cavity.
The present invention provides composite materials with low shrinkage and high surface hardness, and a method for preparing these composite materials. Within certain ranges of the invention materials are provided which may also be extruded from a dental syringe designed for this use.
Composite materials for dental use and general methods for making them have been known for many years. See for example BP1401805, U.S. Pat. No. 3,740,850, U.S. Pat. No. 4,215,033, and U.S. Pat. No. 3,801,344. These patents essentially describe mixtures of acrylate resins with glass or various metal oxides as filler and claim, for example, the advantages of natural colour and desirable hardness. Although an improvement at the time, these early materials were still relatively week and had low resistance to wear. Improvements in surface hardness were achieved when milling procedures improved and fillers with smaller particle sizes became available. Attempts were also made to use different types of monomer, for instance a silane containing monomer as in U.S. Pat. No. 4,504,231, or a mixture of monomers as in U.S. Pat. No. 5,730,601. However the shrinkage of these materials still remained too high, at around 2 to 3 percent by volume. Further improvements were made by the inclusion of two or more types of filler particle, for example a conventional glass fillers with particle size ranges of 0.5 to 40 microns and 0.2 to 15 microns, together with a fine filler with particle size in the range 5 to 150 nano meters, as in U.S. Pat. No. 4,649,165. These materials however still have the problem that it is hard to incorporate sufficient filler into the composite to obtain the desired hardness. Attempts to overcome this problem have also included the use of surfactants, as in U.S. Pat. No. 4,374,937. However there remains the problem that use of too much conventional filler material in a composite leads to a stiff paste which is hard to manipulate, and ultimately to a dry and non-cohesive mixture. It is desirable to be able to extrude a dental filling material directly from a syringe into the tooth cavity, and this is not possible with such stiff pastes. Such pastes when cured typically have a yield strength around 130 MPa, a surface Vickers hardness of about 70, and a volume shrinkage between 2.5 to 3 percent. The yield strength is the maximum load that may be applied to a material before permanent deformation and damage occurs, and it is desirable that this is a high as possible. A high surface hardness is needed because this reduces abrasive wear of the material, while low shrinkage is desirable in order to minimise gap formation around a filling.